text:
Why does the Airbus A320 series seem to rock left and right in level flight?
I have noticed on all Airbus of the A320 series that I have flown as a passenger (which includes the 319, the 320 and the 321), that whilst in level flight, the aircraft has always a longitudinal axis rocking motion. It is very low but if you focus on the horizon from the window you can actually notice the aircraft rocking left and right. Almost not perceivable but it is actually there! This does not happen on the A330's or A340's
Is there any particular reason for this type of behaviour?

Probably some degree of Dutch roll (the same oscillations should be seen on the yaw axis), due to yaw damper less effective on shorter aircraft. The A319 seems to be known for that.

Interesting. The fact is that on aircraft of comparable role and size such as the 737 this effect is not noticeable.

I am not a pilot or aeronautically trained, but I understand that essentially all airliners have a dynamic yaw instability aka Dutch Roll and have stabilisers designed to address this. ANY feedback error correction system requires SOME error signal to derive the feedback from and how much error and how it manifests changes with system.
@mins I'm no expert in this area BUT all aircraft are different and it would be very surprising if they all behaved the same. The overall combination of a wide range of dynamic forces leads to the end result.
As I recall (probably from Peter R March's very interesting 'The 747 story'  Dutch roll was not a major factor in large aircraft until the larger jets and they lost a prototype with an experienced pilot inexplicably. Subsequent investigations and close calls showed the cause was a dynamic yaw-roll coupling. AFAIR this is caused by the high yaw-induced rolling moment of swept wings and as these are standard fare as speeds rise all large/fast modern airliners have this issue and have a yaw damper to make it unnecessary for the pilot to deal with it.
Without a yaw damper, the vertical tail would need to be much larger to provide enough damping. This costs weight and drag and creates problems of its own due to the location of the tail upwards of the line of symmetry. When the aircraft yaws, the wing creates a rolling moment. This rolling motion causes a drag difference which, together with the directional stability of the tail, swings the aircraft's tail around to the other side, and the process repeats in the opposite direction.
It may be, and I'm 'mentating' this with no proof at all, that they are up against a regulatory barrier and that correcting the problem may lead to them having to do something else expensive. eg early British cars were long stroke not for any technical reason but due to taxation advantages as taxing was based on bore AREA. For a given cc size bore area was reduced by increasing stroke. This limited achievable RPM. Japanese engines had no such limits and I remember our amazement in the early days of Japanese motorcycles at the under-square high revving Japanese motorcycles - which gave them a power per cc rating advantage.
Similarly some America's cup yachts at one stage had their ratings improved by tacking light metal sheeting on their decks. This made NO difference whatsoever to performance.
As an example of stabiliser interaction in another field - many modern cameras have inbuilt motion stabilisers. These need to be turned off when the camera is used on a tripod - or else the compensation system causes the camera to "hunt" about the essentially perfectly stable tripod. I've read of people who had major blur issues through failing to realise this.

Thanks for answering! The stabiliser is actually called a roll damper, and the first one was installed in the Me-262. A small yaw oscillation was even present in early propeller aircraft, because the vertical tail is too small in order to minimize drag. It was so bad in the Convair 880 that the aircraft had to be withdrawn from passenger service.

I've noticed this on Boeing widebodies, e.g. 767, moreso than on any Airbus types. However it isn't a design "thing" with the aircraft as such, whether Airbus or Boeing, but is an interaction with the autopilot and the current weather. Widebodies have more inertia to overcome, so the cyclic corrections come further apart unless the crosswinds are particularly strong.
If the aircraft is tracking through a fairly strong crosswind at altitude, it makes more frequent and more enthusiastic course corrections. In Europe that would be on north-south sectors more often than west-east sectors, since the winds at altitude are predominantly westerlies. How aircraft deal with this is down to autopilot function - the point at which it "decides" to make a correction, its "hysteresis", to borrow a term from elsewhere. If these corrections are continuous then it manifests itself as "rocking" - it's just the autopilot working hard at maintaining a heading.

I am real world pilot and flying on the A320 Family.
These bank variations are caused by calibration between the yaw roll and the flight computers. It`s in case of stabilization and recalibration. It's important to hold the aircraft right on track and to maintain a stable information exchange between the board computers and the rudder section sensors.
In normal flight mode (good weather, calm winds, stable horizontal flight without any changes), the aircraft banks a maximum of 4 degrees +- alone to calibrate (more if needed in case of turbulence etc.) and in order between flightdeck and computers a maximum bank of ca. 30 degrees +- (input by A/P or Pilot).

Welcome to aviation.stackexchange.com! I don' t fully understand your last sentence, can you clarify?

What you are most likely observing is the behavior of the AFCS tracking a given course and making corrections as the aircraft drifts to the left or to the right of the course.  It could also be due to turbulence, wind shear or as described above a dutch roll flight characteristic.


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Why is thrust vectoring not used on commercial jets?
Is there a particular reason for which thrust vectoring is not used in airliners as it is with military aircraft besides weight and complexity factors?
I understand that on military aircraft maneuverability is a core component but, by implementing this technique also on commercial airliners, wouldn't there be any benefit? For example one thing that comes into my mind would be reduction of stress on the tail of an aircraft at high speed/high altitude turns, thus potentially reducing maintenance.
Another option would be increased safety, whereas if maneuverability would be compromised by loss of the tail of the plane (for example American Airlines 587), you would still have some sort of control, even though reduced, on the plane with directional thrust.

weight and complexity is a very big factor for deciding to include something or not, also what would happen when (not if) it fails

True. But if it fails there would be redundancy. At least that's my point of view.

@FabrizioMazzoni; "fail" doesn't necessarily mean "stop"; if everything would simply stop when they failed, our lives would be a lot simpler. One of the horror failures in flying is runaway trim, which is where the trim motor doesn't stop, and moves the trim surfaces to their maximum. Not fun.

Agreed with falstro. Putting vectored thrust on airliners would probably cause more accidents than it prevented. Also, the backup yaw authority can already be obtained with the engines by using differential thrust.

Asking if there's any particular reason besides weight and complexity is a bit like asking if there's any particular reason not to eat cyanide, besides it being poisonous. Once there's a significant reason not to do something, there's no need to look for others.

Can you quantify the stress on the aircraft tail during a 15 degree banked turn at FL370 with a mach number of 0.80 ?

Thrust vectoring is for flying outside the "normal" envelope (which is characterized by attached flow). Airliners should really never leave this envelope, so they are perfectly fine with regular control surfaces.
If you want to add thrust vectoring, it would make most sense if the engines are at the back of the airplane. On most airliners, it is really better to put them on and ahead of the wings, because in this location they help both with flutter damping (mass ahead of the elastic line helps) and bending relief. Putting the engine mass right where lift is created is better than carrying stresses all around the airframe, which would be the case with rear-mounted engines.
The redundancy point is valid, but it would be more helpful to have redundant control surfaces, and this is exactly what airliners have. At some point, every airplane has to come down for a landing, which requires to throttle the engines. No thrust, no control!
Most extreme case: If one tail surface breaks off, I wonder if thrust vectoring would be up to the job of trimming the aircraft even with cruise thrust. Not only have the raw forces to be sufficient, the reaction time to control setting changes has to be quick enough to suppress oscillations. I am sure, however, that thrust vectoring will certainly not suffice in all flight phases.
Thrust vectoring is perfect for situations at high angle of attack where you quickly want to point the nose of your aircraft at your adversary, so you get lock-on first. This is quite different from what an airliner has to do.

Do commercial airplanes actually have redundant rudders? I don't think I've ever seen a jet airliner with redundant rudders (or at least haven't noticed it if I have seen it.) I'm assuming that the rudder was the primary control surface that the OP was referring to, since that's the one that separated in the crash he referenced (actually, the entire vertical stabilizer separated because of excessive rudder input.)

@reirab: Yes, indeed. If you look closely, the rudder is split in a lower and upper part, and the same goes for the elevator on each side (inner and outer part, of course). See http://training.deicinginnovations.com/wp-content/uploads/2012/04/IMG_3178.jpg for an example.

@fooot: The smaller ones compensate this with redundant linkages and actuators. This is a requirement for certification.

What about using "Reaction control system" or thrusters on commercial airlines is it feasible?

@SwiftPushkar. In principle yes, but for what purpose? If you think you have a good idea, why not open a new question?

@Peter - I am just enthusiast about the aviation and honestly don't have much know-how about the topic. My knowledge is limited only to tv documentaries shown on tv. I will try to ask the question.

@Peter - What I was thinking about is can these thruster  be employed in case of emergency like freely falling plane? atleast  to slow it down somehow to minimize the damage to passengers when falling to ground.

@SwiftPushkar My answer would go along the lines of the one here about giant parachutes for airliners. Feasible yes, but economically not worth it. This would be similar to the landing thrusters on Sojus capsules. Go ahead, ask the question. We did not have this question here, as far as I can remember.

Thrust vectoring enables extreme maneuverability, airliners don't need that. Personally I don't want to be subjected to 9 Gs on the redeye from JFK to LHR! It's also very expensive technology to build and maintain, so your airfare would go up substantially. You also need a huge amount of power to make it work, so you'd have to fit afterburning engines to your commercial aircraft, which isn't workable.
Anyway, differential thrust is already an option on airliners and other aircraft which have engines outboard on the wings. A B-52H lost its tail in the 1960s and made it back, the pilots made good use of differential thrust. UA232 used differential thrust after they lost hydraulics.
So it's a technology that is expensive to implement, and isn't going to help prevent an accident.

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Do the captain and first officer log the same hours for a flight?
On planes that have a captain and a first officer, how are flight hours logged and split between the two?
Are the hours split between the Capt and First Officer based on who is physically piloting the plane or are the same amount of hours are assigned to both?

So you didn't specify what countries regulations that you are asking about, since it is different in different places.  The answers so far seem to be focused on the FAA regulations.  Is this what you wanted, or are you looking for answers specific to some other country?

Honestly I didn't think about this. Mine was a generic question. Would be interesting if someone could elaborate.

Well, there are around 200 countries in the world, and they all have their version of aviation rules.  Many of them follow ICAO recommendations, but most of those also have their own twists on the rules.  It would be nearly impossible to provide an answer that was true everywhere, so we ask that you tag your question with "faa-regulations", "easa-regulations", etc. as appropriate.  If you aren't sure which one, just include the country that you are asking about in  your question and someone will add it for you.

Both log time, they just log it as different things. This is similar as a student and instructor logging time, but for different roles. The captain is the pilot-in-command and thus logs 'PIC', the F/O is second-in-command and logs 'SIC'.
The pilot who is tasked with controlling the plane is called 'PF' or "pilot flying", and the other is called 'PNF' or "pilot not flying", or "pilot monitoring". This doesn't affect who's PIC and who's SIC though, and if I understand correctly, it's not generally logged who's actually flying.

Ok so there is no distinction between who is physically flying the plane. The same amount of time will be awarded to both and differentiated based on the role you have.

Both log time yes, but when applying for a job, the amount of time you have logged as 'PIC' rather than 'SIC' becomes highly relevant, but then you also look at the amount of time on single and multi, and whether turbine or piston as well. When simply talking about how many flight hours someone has, you usually do not worry whether it was PIC or SIC or FI (flight instructor) or whatever.

and these days most of the time the autopilot is actually flying

@ratchetfreak; well, but the PF is controlling the autopilot. It depends on how you look at it; in a FBW aircraft the pilot is just asking the computer, pretty please move the control surfaces, even if the autopilot isn't active.

Actually, an SIC serving as a PF can still log PIC time for that portion of the flight, see my answer below for details. Many do, precisely because PIC time (especially in a multi) is so valuable.

@alexsh: The SIC can only log PIC time while PF if they have the type rating for the aircraft as well. From what i understand, many airlines don't pay for the SIC to get their types.

@Lnafziger: This is absolutely true. All of my buddies working for the majors are type rated though (even as FOs), even though some of them paid for the rating themselves.

@roe: FBW makes absolutely no difference. 777 is flown just like 767 though the former is FBW and the later is not (Boeing does not implement artificial neutral stability/autotrim the Airbuses have).

@JanHudec I am aware of that (and it was actually exactly my point), I just compared saying that the autopilot was flying the plane (rather than the pilot) to FBW, since the autopilot rarely does anything the pilot hasn't instructed it to do.

To expand on the previous answers (and offer a small correction), CFR 14 part 61.65 states that:

(e) Logging pilot-in-command flight time.
(1) A sport, recreational, private, commercial, or airline transport
  pilot may log pilot in command flight time for flights-
(i) When the pilot is the sole manipulator of the controls of an
  aircraft for
      which the pilot is rated, or has sport pilot privileges for that category and
      class of aircraft, if the aircraft class rating is appropriate;

This has been interpreted by the FAA on several occasions as to mean that an SIC can log the time as a PIC when he is the one flying the aircraft. You can look for 'Legal Interpretation # 92-46' where this very same issue is addressed (they even specifically state that both the PIC and SIC can log PIC time in such cases).
Otherwise, as others have already mentioned, an SIC can log SIC time when he is a PNF and is performing the duties of an SIC (provided the type or operational certificate requires a two person crew).

You might want to mention that an FO logging PIC might make the logbook look good, but no airline will accept that time in an interview and they will question it.  As such, particularly because this Q is about captain / first officer logging time you might want to mention the FAR 1.1 definition of PIC which is the only PIC time an airline cares about (and you will need to be able to separate this out of your logbook for an interview if you have been logging your PF time as PIC)

@casey: The airlines can of course, set any requirements for their hires but it is absolutely legal for an appropriately rated pilot to log PIC time while being a sole manipulator of the controls. FAA has also made its opinion clear about the difference between being a PIC and logging PIC time. According to your very narrow interpretation of PIC in the case of an instrument instructor teaching a student only one of them can log PIC. FAA thinks otherwise and CFR 14 61 reaffirms this.

My narrow interpretation is specific to the very narrow Q concerning CA/FO which implies airline ops.  I have not said anything about flight instruction or the sharing of PIC time via sole manip / acting pic that is often used in multi training.  The question is specific to CA/FO ops, and that is all my comment addresses.  One thing I didn't mention is often the FO lacks a type in the airplane and cannot log PIC anyway.

@casey alexsh and you are both right on this one.  The airlines do not determine what is legal for a pilot to log, and the SIC can quite legally log PIC time when they are sole manip of the flight controls as long as they are typed.  When they go to apply for another airline job, their application will specify what they accept as PIC time, which is usually different than the legal requirements.  This doesn't change the fact that they can log PIC time in their logbook though.

This is well covered by the 14 CFR 61.55 aspect of this discussion:
Does a Second In Command have to be landing and instrument current like a Pilot In Command does?
but a basic important detail is that an SIC has to be required for the operation and you have to be qualified to act as such in order for you to log SIC time.

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text:
How are credit cards processed during flight?
If telephones/smartphones/tablets are not allowed on planes, how comes when you buy duty free items from the cabin crew and pay by cc they will use the card reader?
I suppose that in order to remove credit there must be some sort of connection to the various credit cards circuits in real time in order to achieve this?

It will either go through the airline's existing air-to-ground system (like the seat-back phones you pay $5/min for) or they've got a store-and-process-later arrangement with the card companies.
(Just because you can't use a phone doesn't mean the airline can't, they just follow certain conditions better)

There are some questions on this site that explain that mobiles are banned more because they would not actually work anyway than because they would interfere with anything. They do interfere with earphones (induce noise), but only if closer than about a metre and the passengers are further from the pilots than that.

Mobiles are banned mainly because they have not been approved. FAA/EASA etc. take the opinion that anything attached to or actively working in an aircraft must be conclusively proven to be safe. That's why your airplane, it's parts, maintenance and accessories cost so much.

Negative. This question (that helpfully came up in related) says "… which is why the Federal Communications Commission (FCC) — not the Federal Aviation Administration (FAA) — banned cell use on planes.". FAA and EASA don't have problem with mobile phones anymore. See also http://www.wired.com/2013/10/faa-ban-lifted/.

Cell phones don't work properly because tower antennas are directional (any energy sent upwards would be wasted normally) and the handover process between cells is fairly slow (it needs to appear seamless, even in crowded networks). The network they use for in-flight service is most likely a satellite link.

This answer is wrong. One of the other answers should be marked as the accepted answer.

Presumably, they use an off-line transaction. The reason your credit card has raised numbers is that, in the old days, before ubiquitous data connections, a card transactions used a machine like this

to transfer an imprint of your card details onto a form, using carbon paper. You would then sign the form and the merchant would send it off to the bank to get paid. It wasn't possible to verify in advance that you had enough money in your account but the deal was that the bank would honour the transaction and then come after you with a big stick.  The point of my mentioning this is that, although we're now very used to the idea that the merchant talks to your bank while you make a card transaction, this wasn't the case even 20 years ago.
Today, it's still possible to make a card transaction without verifying in advance that the card has sufficient funds, so that you can still buy stuff on your card even if the merchant's phone line or internet is broken. In fact, the last-case fall-back is to go back to the swipe machines and carbon paper.

Additionally, if you look at the bottom of almost any receipt you sign, you're actually signing a statement saying that you agree to pay that amount as per your cardholder agreement.  So even if the amount would take you over your credit limit, the card company can still hold you responsible for the amount.  And possibly charge you fees (I'm not sure about that one, but it wouldn't surprise me).

What is it called?
Buying stuff on an airplane during flight is know is in-flight commerce (IFC).
How it works?
Credit cards are swiped via wireless handhelds on aircraft but the transactions are processed when the aircraft gets on the ground.
Limitations
Because of this billing mechanism – which sometimes results in fraudulent transactions – there is a ceiling of the value of items that can be comfortably sold today.

Read more here and here.

So what prevents a bad actor from using a seemingly valid, but bogus credit card to buy drinks? Do the offline systems correlate seat position to card use to follow up on fraud later?

HI @Farhan -- I'm surprised: don't SOME transactions go via whatever system the aircraft uses, for, air-to-ground telephoney?  Visa and MC ar both incredibly down on offline transactions these days; I'm surprised it's possible.

Freiheit - with genuine offline credit card transactions (I"m astounded to hear these are still possible today, in any context) - quite simply nothing, zero, prevents fraud.  For example, a very very rare example is that on the freeway system in Spain, when you pay a few bucks toll using a card (unless the technology has changed just lately), it's an offline transaction. "everyone" knew this and you could scam it with, basically, anything that "looked like a credit card" heh. There is no, zero, protection against fraud w/ offline card transactions.

@Fatster, I assume that it depends on the company running the concession (although it makes sense they are offline transactions, as tolls are usually in the middle of nowhere). However, it would be really simple for them to attach the plate number (or just a photo) to the transaction id (clear them nightly, after transactions have been processed). As the fine if being caught will be much more expensive than what they are ripping off, it's surprising they are so mean to do that. OTOH, if the company doesn't bother to prosecute them is a sign that it is uncommon enough not to be worth.

@Faster How is that supposed to work? I have heard about credit card fraud on tolls (mainly with stolen credit cards) but how does that work with something that merely “looks” like a credit card?

@Fatster: It would be surprising if the card transactions went through the cockpit's communications systems; I'd expect that to be isolated from the plane's other systems.  Of course, some planes have wifi available to the passengers, and in that case card transactions could use the same.  Otherwise, in principle the system could keep track of the seat number of the passenger making the transaction; if the transaction is later rejected, the airline already has verified contact info for the passenger and could probably find them and demand payment, or at least blacklist them.

@Fatster: Oh, or did you mean a system like Airfone?  That could work, but I haven't seen those on a plane in several years.

I worked with the credit card processing at one of the bigger airlines a few years ago. We used handhelds that stored the info from the magnetic strip of the card. The transactions were uploaded to our back office system and then to the payment service provider after the flight landed.
The only "hard" protection is the modulo 10 checksum of the credit card number (http://en.wikipedia.org/wiki/Luhn_algorithm). The fact that the airline knows exactly who is on a flight also makes it much harder to get away with this type of fraud.
During my time at the company I don't think I heard of a single case of attempted credit card fraud (that doesn't mean there weren't any). I think theft is a much larger problem in on-board retail.

Transactions are offline.  The data is sent to the payment gateway on connection between the POS and wifi/GSM network on the ground.  Fraud happens, a lot.  Airlines have dedicated departments to chase these failed payments with varied success.
One Dutch airline, once the card ceiling was crossed (e.g. upgrades for a family might cost over €1000 which might be the ceiling limit set) would have the captain make a call to have the card payment cleared.
You're welcome,
Mr J.

[citation needed]

? What do you mean?

that you do not provide any source for the claims you make. for example, who says that "fraud happens, a lot"?

I am the source.  It is the world I work in.

In addition to the methods other answers have already explained in detail (i.e. just running the transaction offline and hoping for the best, using existing in-flight connectivity including ACARS and/or other forms of SATCOM or VHF), when the payment card features a smartcard chip (or exists as a digitized version of it in a mobile wallet), there are options to make offline transactions a bit less risky for airlines:
Offline card authentication
Chip credit and debit cards are often capable of authenticating themselves to a card payments terminal using public key cryptography. While this does not solve the issue of fraudsters trying to use lost or stolen cards, it does catch completely fraudulent cards (i.e. those created by fraudsters from scratch in whatever way, or cards with a modified number, potentially copied over from another stolen card).
This method works for mobile wallets such as Apple Pay or Google Pay as well.
To augment this, airlines can use a list of known-bad card numbers (e.g. those having been involved in fraud in the past at the airline directly, or a list provided by their payment service provider or the payments network).
Some chip cards also support PIN verification on the chip itself, which further prevents the chance of fraudsters being able to use a lost or stolen card on an airplane, even if there is no real-time connection to the bank to verify it.
Offline transaction authorization
Some cards (or to be precise, their issuing banks) go a step further. It's possible for the chip to actually create a cryptographic approval of a given transaction directly. This usually includes card authentication and, if applicable, PIN verification, as explained above. But in addition to that, the chip can also take previous transactions and other conditions into consideration.
Commonly, issuers will allow a certain number or aggregate amount of transactions to complete directly on the card, after which the card will refuse to take on more offline transaction risk and require to be "refreshed" as part of a regular online chip transaction (at which the bank gets a chance to reset the various risk counters used, or alternatively block the card, should it have been reported stolen or overdrawn).
This method usually shifts the risk from the airline to the card issuer: Unlike non-chip offline authorizations, which generally happen at the merchant's (i.e. the airline's) risk, issuing banks generally have to honor offline chip transactions.
However, since many cards don't support this mode (or the limits might already be exhausted from offline transactions before boarding), many airlines won't insist on this transaction mode completing successfully. It also generally isn't possible with mobile wallets, since the same card can be present in multiple devices and it wouldn't be possible to synchronize the offline transaction counters across them without internet connectivity, so the risk to the bank would be potentially uncapped.


text:
Do the effects of turbulence change when banking/turning compared to level flight?
Are the effects created by turbulence on the aircraft different when the aircraft is banking or in level flight? By logic I'd say yes but I would like some technical and practical explanation.
When in level flight I can imagine that the aircraft is moved laterally or vertically. What happens when these forces are applied from turbulence when  banking?
What are the consequences of turbulence having the aircraft banked, for example, on the lift produced by the wings?

wake turbulence can introduce a roll when you are more in one side if the double wake vortex

Turbulence affects aircraft in the same way whether it is banked or not.  The difference however would be the end result of the turbulence.
For example, if an aircraft were to encounter turbulence strong enough to roll the aircraft 20 degrees to the right, the end results would be a 20 degree bank if starting from level, but 45 degrees of bank if they were already in a 25 degree banked turn.  If this were to happen it puts them closer to their stall speed because of the higher bank angle, and if they were already going slow and the airplane is at a low altitude, it could be a problem.
Fortunately, most turbulence strong enough to cause this while at low altitude is caused by the wake turbulence of other aircraft or microbursts, and we have pretty good procedures in place to keep it from being a problem these days.  If they aren't at a low altitude it matters even less because there is altitude for the aircraft to recover even if it does get into an unusual attitude.

Just to elaborate on this, when a plane is banked, part of its lift is used to provide the horizontal force necessary to turn.  This means the plane need to generate more lift to maintain altitude.  This puts more load on the wings, potentially putting the plane closer to it's design limits.  The addition of turbulence could potentially cause a problem, but in truth, a bank needs to be extremely aggressive to cause significant wing loading.  All of this would only come in to play in much more severe turbulence than you'd ever want to encounter.


text:
How can cold air inflow in high tropospheric layers create a depression?
I’m studying meteorology for PPL and after learning the big differences between cyclones and anticyclones, I came across a question that flipped all my knowledge on its head:

What pressure pattern may result from cold-air inflow in high
tropospheric layers?
A: Formation of a low in the upper troposphere

I’ve spent hours on all facets of this question and I still don’t understand how cold air inflow can create a low in the upper tropospheric layers.
Apparently this cold air inflow at high tropospheric layers triggers a through, which would be a catalyst for ts and cb.
But at the same time it doesn’t make it clear how cold air can create a low, cyclone, which is actually characterized by rising warm air that cools.
There is a variation of this q that talks about what weather conditions should be expected in an upper level through and it suggests that‘s development of showers and thunderstorms.
But I thought throughs in lows happen at ground level, because air rises in lows.

These days when we can just check the near-term forecasts on the internet, it seems odd to have that kind of detailed theoretical meteorological question on the PPL exam!

I agree, and what’s worse is many questions only make more sense once you tap into ATPL resources.
It’s either books need updating or questions need to be less ambiguous.

Best book on aviation weather theory, bar none, is the manual published for Royal Canadian Air Force pilot training. It's a fundamental part of the ATPL knowledge base. Extremely well written. https://publications.gc.ca/site/eng/9.652598/publication.html#:~:text=The%20Royal%20Canadian%20Air%20Force,taught%20in%20a%20classroom%20setting.

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text:
What is the relation between solidity and lift coefficients?
I've seen this graph in a couple of places. I'm not sure what it's supposed to mean, as the formulae I've seen in the papers cannot be found in any standard book or reference.
Please let me know if there is some solid reference for this, or if you could explain what exactly this graph represents.

Could you provide a link to a paper that it is from?

@MikeY One of them: https://core.ac.uk/download/pdf/37995514.pdf

What are l and t and what are the coefficients that we are getting the ratio of?

@MikeY t/l is solidity, and I'm not sure about what those coefficients are. One of them is the lift coefficient that we normally use.


text:
How does an aircraft descend without its nose pointing down?
I have seen many cockpit videos of airplanes landing, and nearly none of them have their nose down for losing altitude. How does this happen? and how does an airplane, such as A320, descend without its nose pointing down?
And how does this happen in a controlled manner?

Of note, Air France Flight 447 on a Boeing A330 did exactly this for 38,000 ft and crashed.

Which way is the nose pointing on a brick? Throw a brick in the air, and it descends just fine without caring which way is is its nose!

"The paradox of the glide. By pointing the nose down less steeply, you descend more steeply. By pointing the nose down more steeply, you can glide further." https://books.google.com/books?id=CPdDju21zt0C&dq=isbn:9780070362406

A good flight simulator will give you a great 'feel' for flight. If you get the chance, give one a try.

@Nelson "Boeing A330"? Now that would be a major merger!

@GlenYates Ha!  Oops, not an aviation regular... but yes... not Boeing...

Imagine the plane reducing speed to 0 in air. With the nose pointing forward, would it then hover?

You might notice the next time you're in a plane and it's landing - the back wheels almost always touch down before the front one.  Sometimes they all touch down at the same time, but one thing you'll (hopefully) never see is a landing where the front wheel touches down first.  (Unless it's one of those old-timey planes with the one wheel in the back.)

@Nelson haha, yeah, that was definitely an Airbus. Which was actually quite relevant, as that case was a classic example of potential pitfalls of pilot over-reliance on technology and not understanding how to fly the plane when the technology isn't doing it for them.

If all it would take for an aircraft not to fall out of the air is to put its nose up, a flight would be a lot cheaper.

A plane descends when it does not have enough thrust to maintain its altitude. A plane can descend with its nose pointed up or down so long as there is not enough thrust to maintain altitude. Altering the pitch of an aircraft is used to control airspeed: pitching up slows an aircraft down (and may cause a climb if done from a level attitude) and pitching down generally speeds up an aircraft (and causes a descent if done from a level attitude). So if you want to descend at a slow speed you reduce power and pitch the nose up.
Since you want to land at a slow speed you will need to pitch up and slow the aircraft down while reducing power to descend nicely. This article covers it nicely in terms of flying an approach. It's also covered in this question.

Shouldn't it read "when it does not have enough lift to maintain its altitude?

@Tashus: no, not in the aerodynamic sense. An airplane that is not accelerating -- i.e. is in a steady state -- whether climbing, descending, or in level flight, is generating exactly the same force in lift as gravity causes in weight. The two forces are balanced. It is thrust in excess of that needed for level flight that allows for a climb, and thrust less than that needed for level flight that causes a descent.

I'd say this answer, while "good enough", glosses over/is imprecise regarding some details. E.g., from level flight, pitching up will generally cause a momentary climb, but if the airplane's moving slowly enough, will then enter a descent unless power increases. Similarly, a descent at slow speed may often be accomplished simply by reducing power and maintaining, rather than increasing, pitch angle. It's true that airspeed may be controlled with pitch angle, but the implication that a given pitch angle will always produce the same given airspeed is incorrect (and almost certainly unintended).

Also, gliders ("thrust"?).

@PeterDuniho that isn't the aerodynamic sense at all - what you've described is the usual parameters of a fixed-wing flight, but excludes without explanation many other things that are aerodynamically possible. An aircraft with surplus thrust can enter a dive and thus lose altitude; a glider with no thrust can, in the right updraft, gain altitude. In all these cases, in the aerodynamic sense it's the lift that determines the aircraft's altitude. Airspeed just happens to be one of the most straightforward ways to change lift, and thrust is a good way to affect that.

@Will -- are you suggesting the magnitude of the lift vector controls the climb rate or sink rate?  Not so; see links under my answer.

@Will --Lift is less in a sustained steep climb than in a sustained descent at a moderate angle.

As someone who has done lots of low-level passes and a fair amount of aerobatics on gliders, I  will need to be explained where the "thrust" came from in the parts where the glider was going up.

Simultaneously intrigued at the idea that pilots use thrust to control (rate of) altitude (change), and befuddled that you never explicitly mention that lift from a wing is proportional to its airspeed... Which is the root of the phenomenon of "sinking while not pointing down".

@Will: I specifically stated that the scenario I'm describing is unaccelerated flight. This is the first, simplest configuration considered when a pilot learns about aerodynamics, and is perfectly sufficient to address the comment. If you'd like to present a dissertation on the broader scenarios available, that's fine, but  I would not say that Stack Exchange comments are the best forum for that.

@Martin: important to the answer here, and my first comment above, is the constraint that we are discussing unaccelerated flight. I.e. all of the forces are in balance. I even said as much in that comment. Your "low-level passes", including "the parts where the glider was going up", in a glider are most certainly not unaccelerated flight, and so have no bearing on this particular discussion (indeed, it serves only to obfuscate and confuse)

@GrimmTheOpiner: "lift from a wing is proportional to its airspeed" -- not really. Lift is proportional to angle of attack. Lift is proportional to the square of the airspeed, but it's important to keep in mind that airspeed in turn is a function of thrust and drag, and high angles of attack produce higher induced drag from lift. If lift's proportionality to the square of airspeed is literally the root of "sinking while not pointing down", then it's also the root of "flying level while not pointing down" and "climbing while not pointing down", making it not explanatory at all.

@PeterDuniho: the words "accelerated", "unaccelerated", and "balance" appear in your comments, but not in the answer. As the accepted answer and the one with the most votes, it is the answer and not my comment what "obfuscates and confuses".

@MartinArgerami: well, I can't disagree with the factual statements about what the answer fails to include. After all, I did in fact point out that this answer leaves out important details, and in so doing is imprecise. But, as far as obfuscating and confusion goes, the answer addresses the question that was asked far better than your note about accelerated flight in a glider does.

"Since you want to land at a slow speed you will need to pitch up and slow the aircraft down" Well, yes, that's certainly one reason. However, wanting to land on the main gear rather than the nose gear is also a pretty important reason for the flare in a tricycle configuration. :) You can land with the nose lower, but bad things tend to happen.

When an aircraft ascends or descends, its inertial velocity vector (let's simplify by saying the ground is inertial) will be pointed upward or downward. The angle with respect to the horizon, or ground, is called the flight path angle ($\gamma$).
There are two more angles relevant to this story:

The pitch angle ($\theta$) is the aircraft attitude with respect to the horizon.
The angle of attack (AOA) ($\alpha$) is the air flow incidence against the aircraft.

When aircraft is not turning and there is no wind with respect to the ground, the relationship between the three angles are:
$$\theta=\alpha+\gamma$$
So as you can see, the plane can descend with a positive pitch angle. Or it can do level flight with a non-zero pitch angle. Pitch angle has no direct relation to the flight path.

Image: https://www.basicairdata.eu/knowledge-center/measurement/in-flight-angle-of-attack-usage/

The extremely coarse "explain it like I'm 5" answer:
You know how they talk about in school, how the curved top part of the wing has faster air across it at lower pressure, and that creates lift?  They make a big deal about the Bernoulli Principle and how it's not just a wedge pushing through the air. Yeah, they didn't tell you the whole story.  Airplanes also do the "wedge" thing.  They make lift by angling the wing upward.  In fact if you've built a paper or stamped balsa wood airplane, you may notice that it flies even though its wings are obviously flat.  You could build a 737 with flat wings and it would fly.  It would not get very good fuel economy.
The faster a plane goes, the better the wings use the Bernoulli Principle. The slower, the worse; so at slower speeds they must tilt the wings upward and using them in wedge fashion.
The nose is connected to the wings, so they must lift the nose to tilt the wings.

nose is connected to the wings citation needed.  ; )

Really short version:

"How does an aircraft descend without its nose pointing down?"

By descending with its nose pointing level or up.
Thrust is not required to do this.
Here's an example:

Now for a tiny bit more detail:

How does an aircraft descend without its nose pointing down?

By flying along a shallow glide path at a moderate to high angle-of-attack.
Angle of glide path (relative to airmass, and expressed as a negative number) plus angle-of-attack minus angle-of-incidence equals pitch attitude of fuselage.
The resulting pitch attitude may be positive (nose up), negative (nose down), or flat, depending on the values of the other three angles involved.
If we ignore the angle-of-incidence, which is rather small in most airliners, then we can say that to a first approximation, the fuselage will be in a nose-up pitch attitude whenever the angle-of-attack is larger than the angle of the glide path measured with respect to the surrounding airmass, which in still air, is the same as the angle of the glide path measured with respect to the ground.
Draw a sketch that shows the wing's angle-of-attack in relation to the glide path through the airmass, and you'll see.  For normal climb or glide angles a heavily-loaded jet needs to fly at a high angle-of-attack to generate enough lift when flying slowly.  Also, airliner-style approaches to landing typically involve maintaining enough power to create a rather shallow glide path.  The result is a nose-high pitch attitude.
In a descent, gravity is supplying power to the aircraft, which decreases the thrust requirement associated with any given airspeed, i.e. which decreases the thrust required to offset drag and keep the airspeed constant, i.e. which increases the airspeed associated with any given thrust setting, all compared to level (horizontal) flight.  (The opposite happens in a powered climb.)  But this "boost" from gravity doesn't depend on whether the aircraft's pitch attitude is nose-up or nose-down.  It only depends on how steeply the flight path through the airmass is inclined downward.
One way to more comprehensively answer this question would be to create a 3 x 3 or 4 x 4 grid of 9 or 16 panels.  Each column would show a fixed angle-of-attack (and therefore also a fixed lift coefficient and drag coefficient), progressively decreasing from left to right.  Each row would have a fixed thrust setting, progressively increasing from top to bottom.  Each panel would have a sketch showing angle-of-attack, direction of flight path through airmass (i.e. direction of airspeed velocity vector), pitch attitude, and airspeed.  Some of the aircraft would have a nose-up pitch attitude, and others a nose-down pitch attitude.
For more, these related answers to related questions:
"Are we changing the angle of attack by changing the pitch of an aircraft?"
"Where is the zone of reversed commands?"

"Does lift equal weight in a climb?"

"What produces Thrust along the line of flight in a glider?"
"'Gravitational' power vs. engine power"
"Descending on a given glide slope (e.g. ILS) at a given airspeed— is the size of the lift vector different in headwind versus tailwind?"
"Is excess lift or excess power needed for a climb?"

Supplemental info that I might add to answer  -A constant pitch "input" will, to a first approximation, maintain a constant angle-of-attack, not a constant pitch attitude.  Even at a constant angle-of-attack there is some variation in airspeed as power varies, but in the opposite direction as you might expect.  For example holding the same high angle-attack you'd normally use for an efficient glide, and adding enough thrust to sustain a 60-degree climb path, means that the airspeed must ultimately decrease, or else the excess lift will tend to make the climb path to curve closer to vertical.

Ctd - During the time this deceleration is happening, you'll need to temporarily decrease the angle-of-attack a bit if you don't want to "overshoot" your target 60-degree climb angle.Recalling of course that less lift is required for a climb than for level flight.  Exactly the same logic holds true in a 60- degree dive, which would be only possible if you deployed some sort of large drogue chute or other similar device to increase the drag coefficient, as long as we're still talking about flying at a high angle-of-attack.

Ctd - see https://aviation.stackexchange.com/questions/40921/does-lift-equal-weight-in-a-climb/56476#56476

Winter project: create the table described above.

The angle at which the nose of the aircraft points has less to do with descending or climbing than your intuition would lead you to think. Airspeed affects the rate of climb, not attitude. The faster the air flows over the wings, the more rapidly the aircraft will climb. Slower air flow causes the aircraft to descend.

The airplane flies because lift created by the wings counteracts its weight created by  gravity. To create lift the surface of the wing must be in relative forward motion through the mass of air. Thrust created by the engine generates the forward motion which is opposed by drag. As long as thrust is greater than drag, the wing can move forward and generate lift. Gravity is universal and operates in all objects. Lift, on the other hand applies only to whatever is attached to a wing. To generate lift the wing must have the air moving around it following certain rules, otherwise it is incapable of generating lift. These rules are conceptualized as the angle of attack or the angle between the cord of the wing and the relative motion of the aircraft. At high angles of attack the wing is incapable of generating lift. Gravity, experienced by the airplane as weight does not follow any particular rules, all objects fall from the sky in the absence of lift. An airplane can have its nose up or down and gravity will still apply and if the lift is less than the weight, the airplane will go down. In fact, during landing most airplanes will maintain a nose-high attitude while going down into the runway.

The wing moves forward only when thrust is greater than drag?  Go directly to jail, do not pass go, do not  collect 200$.  Also be aware that in a climb, lift is less than weight; your answer implies otherwise.  Basically you are presenting an Aristotelian world view here rather than a Newtonian one.

Yes. My simplified view is Aristotelian and and entirely ignores inertia, which also explains why in a climb lift is greater than climb as long as all other forces stay the same. This explanation is much easier to grasp for the uninitiated and is a good intuitive first approximation.

But you are just making things up that aren't the least bit true!  Ignoring inertia (e.g. not being able analyze situations involving acceleration) has nothing to do with it; your "explanation" is completely wrong even for steady-state conditions.

I assume you meant to say in your comment that in a climb, lift is greater than weight. Not true.

How does a rocket climb vertically?

Not with lift.  The notion that lift is greater than weight in a climb has already  been debunked in response to several different questions on a.s.e..

https://aviation.stackexchange.com/a/56476/34686 -- and see attached links

Maybe the answer is that the rocket does not climb with aerodynamic lift. A good starting place for the explanation.


text:
Help ID a WW1 Plane
I purchased a wooden box of glass and nitrate negatives at a flea market in Berlin Germany in 1995. There are about a dozen more in the lot with planes in them.
This image has never been published to my knowledge.
Any ideas on the plane or the pilot?

This image seems to be reversed left-to-right.

Yes it is, I had to choose between the emulsion side and the other - not knowing which is correct.

You can see it's reversed because of the cut of his coat: the left side always comes over the right for men.

About the airplane: It looks like a Fokker D.II.
About the engine: it definitely looks like a Oberursel_U.I.
About the pilot: thanks to @PortreeKid we know him to be the test pilot Werner Emil. We speculate that he doesn't look happy since he is missing a button on his jacket

The missing button is the reason for his unhappiness :)

@U_flow  I'll update the answer

https://fokker-history.com/en-gb/d-ii#&gid=1175965900&pid=17 Emil Werner also seems to be unhappy about a missing button.

@PortreeKid: perfectly spotted  But then this picture has actually already been published...

@sophit yes but faaaaar better quality. Presumably same series and this one is a close up

WOW - the picture I posted is an uncropped glass negative in my position.

Opps, hit enter to soon. I'm so glad I posted this, to see another broader image and a name is just mind blowing, again WOW.

Now does anyone know the photographers name?

It seems to be Emil Werner, not Werner Emil, right? (The problem being that both "Werner" and "Emil" are German first names, but of course either one could be the family name here. The answer has it the other way around than the link.)

@PortreeKid it seems to me that this is not the same series, but literally the same photo. If you overlay them in an image editor, they are identical to the last crease on the uniform, and the face doesn't move the tiniest bit. Negatives may be duplicated (and maybe flipped in process?), so it's likely that the OP got a copy.

IMil I'm just flipping back and forth in 2 windows and it does look like I have a cropped copy of the full picture. I scanned the negative I have and got a 20 meg file. I reversed (flipped) it to match the link and zoomed in on the markings on the casting of the motor and it can be clearly seen and letters can be made out but it can't be read. For a copy of a negative done probably a 100 or so years ago I don't know what to say. If you would like a zoomed in part of my picture I could work on that this weekend. I'll dig up some of the other plains in the collection and post them to.

@GregParker I'm not an expert, so a zoomed in picture will be of no use to me. What I know is that: a) negatives may be duplicated by contact printing, which almost preserves the original quality, b) glass plates used to be produced far longer than most people think (based on this thread https://www.photrio.com/forum/threads/where-to-buy-agfa-6x9-5-glass-plates.62842/ , even in the last decade). And I suppose the Berlin street sellers are not above passing copies as originals to increase price.

The box I bought has about 500 negatives of both glass and nitrate negatives in it and I bought them all for about 30 dollars, also the nitrate negatives were in a paper bag from a developer and the print is all in gothic German, I'm 100 percent certain they are not copies and period correct. Now for the print in question I don't know why they are nearly identical. I do appreciate the feed back. Thanks.

@GregParker that's interesting. But for the reason I've mentioned above, I still think that your photo and that one on Fokker museum come from the same shot. And since yours is cropped, it has to be a copy. Otherwise it would have to be physically cut from a larger original, and why would anyone do it? But it may have been copied back during WW1, to make a postcard or whatever.

I do agree with you that what I have does absolutely looks like it came from the photo from the larger shot and that it might have been for a post card, or maybe for the family of a deceased pilot?


text:
Where is the midspan airfoil located on the 737?
The 737 has a root airfoil, two midspans, and one outboard. Is the midspan located under the fuselage or between root and tip?

You don't need an airfoil under the fuselage. "midspan" is between the root and the tip as the name suggests.

The midspan airfoil is between root and tip, so somewhere along the half span of the wing. Midspan is therefore a misnomer, but sounds better than midhalfspan.


text:
Why do newer engines have such wide fan blades?
Every treatise I read about propellers/fan blades/wings states that long, thin wings are more efficient than wide stubby ones.
So why is it that the blades in high-bypass turbofan engines fitted to commercial airliners -- that account for the vast majority of airmiles flown -- have, and continue, to get wider* as the planes to which they are attached have become progressively more cost efficient?
I would prefer an intuitive explanation rather than a deep mathematical dive, but I'd also like links to any papers that explore this phenomena.
* In both relative -- as a proportion of the circumference -- and absolute terms. Engines have gotten progressively larger in diameter, and have progressively fewer blades.

Two instant thoughts, not sure how relevant they are: 1) those are not wings, they're compressor blades. Their job is to push air and keep it pushed. 2) efficiency for an airfoil is only part of the problem of overall system efficiency.

And they are spinning very fast... Intuitively longer high aspect blades are better a slower speeds.

Fan blade evolution

The historical fan blade had the same form as a compressor blade,
Figure 3 and 4. The weight of the solid titanium blade stopped a wider
cord design. The long and slender shape made it difficult to get the
blades stable enough to avoid flutter (aero-elastic resonant
deformation), therefore mid-blade clappers were needed. These caused
efficiency losses due to shock waves that formed around the clappers.
As fan blades have a nacelle wall that stop tip streams, they don’t
have to have a high aspect ratio to be efficient. As hollow titanium
blades were developed, fewer wide-chord blades could be used. These
are more robust and therefore could be made without clappers.
Gradually the form was adapted to the different flow states over the
blade.

The original questioner requested an intuitive explanation. It's important to note that "We made the blades wider, thicker, hollow, changed the topology, all the time dancing around resonance issues- none of these changes could be made on their own, but together they give a 4% efficiency boost" is code for "You ain't gettin an intuitive explanation"

The improvement in efficiency has two roots:

Higher turbine pressure ratios. While the very early turbojets managed just 3.14:1, the most recent civil turbofans run at 50:1.
Higher bypass ratios. The first turbofan had a 0.3:1 BPR while the most modern ones run at 11:1.

Yes, the same number of more slender fan blades would help to increase efficiency, but they would never suffice to absorb the enormous power provided by the turbine. Like in a wing which needs a certain area to produce the desired lift, engine fans need enough fan area order to create thrust. Since the diameter of the engine can not grow indefinitely, all that area has to be provided within the given fan diameter. A larger fan would have higher tip speeds, and while the current large fans already run supersonically at the tips, the efficiency goes down if more of the fan runs at supersonic speed. Also, the size and drag of the fairing will go up, reducing the gains from a larger fan. Lastly, the fan still has to fit under a wing.
The ratio of the area of all fan blades relative to the area of the fan disk is called activity factor or solidity. With the same activity factor, a fan with more but more slender blades will have more friction drag because the Reynolds number of those blades will be lower compared to a fan with fewer and wider blades.

Long, slender wings impart a small momentum change to a large volume of air. Short,  stubby wings impart a large momentum change to a small volume of sir. Small momentum changes generate less frictional losses so they are always favored for propellers and wings.
However, the centrifugal forces experienced by a rotating propeller blade scale as the square of the prop diameter and if too large, they will tear the blades off the hub. The strength of the blade material sets a limit on the prop or fan diameter for any given value of spin speed, and for high RPM fans the blades must be made shorter and wider (and thicker too) so they do not destroy themselves during operation.

Gliders utilize a long span, short chord wing design to achieve high efficiency as you described. This design enables them to accelerate the necessary mass of air downwards with the least amount of induced drag and skin friction.
The problem with engines is that they also need to accelerate a lot of air but the area constraints are strict: the required area of wing, the blade that is, has to be packed very very tightly and the remaining "deficit" is compensated by (rotational) speed.
As for the trend of fewer but wider blades: Modern materials and manufacturing methods allow the use of wider blades that are better in the sense that an air particle can be more gradually accelerated (backwards) along a wider blade, instead of forcing the same  acceleration in a shorter distance. The profile of the wider blade can be designed to be "gentler" and more efficient.

I would be interested to know what my nemesis finds to be untrue in this answer. As well as the others you have downvoted.

I'm confused by "long, short span wing". How can a wing be both long and short span? Did a word or two get left out before the comma?

Oops, that was supposed to be long span, short chord, glad someone caught it @FreeMan

I see no "nemisis". Your post is interesting; but less than authoratative.

@Buk less authoritative does not justify a -1 vote. Negative votes are for bad answers that contain misconceptions or false information.

I didn't downvote you. But, whilst 'less authoratative' might not warrent it; this does: " and more efficient so to speak." That phrase is meaningless 'so to speak'.

@Buk I didn't imply you did. I agree with your comment on "so to speak" part and removed it. Using such a phrase, however, does not void my answer either. An answer may have a meaningless phrase, but as long as it has its facts right, it's not a bad answer.

I would prefer an intuitive explanation rather than a deep mathematical dive

No equations, I promise!

Why do newer engines have such wide fan blades?

To answer this question, it has to be understood how the blades of a jet engine work (regardless of where they are mounted, i.e. on the compressor, on the turbine or on the stator). In particular, their main task is to bend as much as possible the airflow that passes over them. The following picture (source) shows how the airflow's velocity changes due to the blade's presence:

Being $\beta_1$ the angle of the airflow entering the blade and $\beta_2$ the angle of the airflow leaving the blade, a good blade design is where their difference (normally termed $\epsilon =  \beta_1 - \beta_2$) is as big as possible, theoretically just equal the angles of the leading and trailing edge. To achieve this, the distance between two blades (called pitch, s) must be within a quite narrow value, around 1 to 0.5 the chord, c of the blade. Intuitively, this can be explained as a sort of "channel effect" that is created when two blades are close to each other and that helps the flow to follow the blade profiles. If the blades are too far apart, this channel effect vanishes and their efficiency too. Anyway they do not even have to be too close (pitch<0.5), otherwise a "blockage" effect results.
Now to answer your question: if the blades went out of the rotating axis just straight with a constant chord, then their pitch s would increase more and more moving away from the axis, losing that "channel effect" and efficiency. Increasing the chord of the blades proportionally to the distance from the axis, simply keeps the pitch within the optimal value.

long, thin wings are more efficient than wide stubby ones

Correct. Simplifying to the extreme, a blade or a wing work pushing air downward and receiving as a consequence, by virtue of the third Newton's law, a net "push" upward, that we call lift. Lift is proportional to the product of the speed at which the blade/wing pushes the air downward and to the mass of air being pushed. By simple energy considerations, it can be demonstrated that this process of generating lift is more efficient if a lot of air is pushed with a small speed. This is basically why a blade or a wing becomes more and more efficient the wider it gets and that's why helicopters and gliders tend to have lifting surfaces with very long span (technically they generate less induced drag).

why is it that... engines have gotten progressively larger in diameter?

The reason should be clear now: the longer their blades are, the more efficient they get. But why then jet engines didn't have a wide fan since the very beginning? Technological limitation: a rotating mass experiences a centrifugal force proportional to its distance from the axis of rotation and to the rotating speed squared. Just as a comparison, a jet engine rotor with its 15'000 rpm sees a centrifugal force which is 2'500 times higher than a helicopter rotor rotating at 300rpm! Structural strength limits the maximum diameter of a jet engine to a couple of meters and this structural limitation was even more strict back in the early days of jet engines: with the evolution of material technology the diameter has become increasingly bigger... anyway we are never going to see a jet engine with a fan as big as a helicopter rotor. Technological evolution also explains the fancier shape of the blades which now can be driven not only by mechanical requirements (it has to be straight to resist centrifugal force, full stop) but also by aerodynamics requirements.

why is it that... engines have gotten... progressively fewer blades?

This is not completely true. An old (1974) CFM56 had 24 blades on its fan, just like a modern (2003) RR Trent 900. An old RR Convey (first turbofan to enter service) had 20 blades, only 2 more than an ultramodern (2006) GEnx. So the number of blades is more related to the producer's in-house technical choices than to an historical evolution.

As promised, no math

ϵ=β1−β2 is an equation :)

@Jpe61: you're right


text:
How can airport elevation be different in various approach charts?
If you see KRDM RNAV Y RWY 29 - airport elevation is 3080.
But in RNAV Z RWY 29 - airport elevation is 3082.
How it can be different for approaches to the same airport and runway?

To add to the correct answer by RTO, a survey was completed on Jan 26, 2019. The airport elevation is 3081.5 (which rounds to 3082 on the chart). See the snippet of eNASR below.
The procedure documentation for the RNAV (GPS) Y RWY 29 was amended for the 28FEB19 effective date (to add the VGSI and RNAV glidepath not coincident note). The results of the survey were likely not yet available for publication when the chart was updated.
The procedure documentation for the RNAV (GPS) Z RWY 29 is similar, but it was updated via transmittal letter (TL) 22-13. It specifically updates the ELEV, among other things. (Link below)

https://www.faa.gov/aero_docs/acifp/NDBR/96980669FE9542F7B95214A1951A1012-RDM-NDBR/P-NOTAM_OR_RDM_RGZ29_AMDT%201B.pdf


text:
Why was the Ka-50 designed with only one seat?
The Soviet/Russian attack helicopter Kamov Ka-50 Black Shark is unique in many ways, one of which being the only attack helicopter to my knowledge that only seats one crewmember. Generally, attack helicopters have two seats to manage the flying and shooting because it's difficult to operate them, and the Ka-50 has a modified version which is the Ka-52 Alligator that has two seats and has been produced in greater numbers to follow that, so why was it only designed with one?

From what I could find, it was determined in tests that it was able to be flown by one one person but it probably wasn't ideal considering they made the Ka-52. Making an educated guess, I presume it could also to have fewer crewmembers per helicopter in order to be able to deploy more helicopters, which was a center point of Soviet strategy.

Ground attack aircraft with 0 seats is pretty trendy right now, guess why?

What you could find?  Where are the references?

Have you watched this video?

Not all design decisions are good ones and it sounds like they may have taught themselves a valuable lesson.

Because they could, and to show that they could.
The Ka-50 uses coaxial rotors, which significantly improve the controllability of a helicopter. It's still not as easy as a fixed-wing, but it frees up some of the pilot's mental resource towards fighting.
The helicopter was also meant to fight entirely with smart weapons. To deploy them, it comes with a helmet-mounted cueing system, similar to that employed on the F-35, even if less high-tech.
The gun isn't fixed or turreted, but uses a computerized aiming system, potentially requiring the pilot to only look at the target and keep the helicopter pointing in its approximate direction.
The F-14 was all two-seat, the F-18 is single or tandem, but the F-35 doesn't have a two-seat version at all. Was that a mistake? Time will tell, but increased automation always prompts an attempt to reduce crew size.
The main drawback of the otherwise very practical coaxial configuration is the complexity and weight of the transmission and dual hubs. Being heavily armored, each crewmember entails a lot of weight. Single-crew configuration requires more pilot skill, but makes up for the extra hardware's weight.
At the time, the attack helicopter market was dominated by Mil with their long experience. Kamov wanted to push a product disruptive enough that it would negate the incumbent's advantage. They succeeded at impressing the observers with impossible-looking feats, and only needing one pilot added to the machine's "rotary-wing fighter" image.
But ultimately, the military never asked for a single-seat attack helicopter. They didn't have a long line of such aircraft, a training program for a combined pilot+WSO skillset, or a particular need for such. It was impressive, but they preferred to spend a bit more for larger engines and a more versatile two-seat configuration.
There are some articles that attempt to describe the development and its motivations, though the bits of information are scattered around quite sparsely.


text:
Where can I find crash information/reports based on the aircraft's Bureau Number?
I am writing a book about my military service. I was working the catapults as a final checker and the F-14 lost the shuttle and went into the sea. Both crew survived.
This was with squadron VF-124 in September-October 1978 during carrier qualifications. Where should I look to find the crash report?

These records are not public. There are enthusiasts that track such things. What sounds like the accident you speak of is mentioned here:

crashed 13.09.1978 into the sea. During a night catapult launch, the a/c launch bar was placed on the lip of the shuttle, vice in the throat. Prior to launch the pilot selected mil. power and indicated to the cat officer that he was ready to go. As the shuttle was released the a/c, in the kneeled position, started down the cat track, whereupon the launch bar slipped off hte lip of the shuttle. Zone 5 afterburner (Z5) was selected at or shortly after shuttle release, realizing that the a/c would not attain flying speed, the crew ejected. With approx. 1,000 lbs removed from the a/c nose and due to the nose strut extending, the a/c pitched up as it left the deck. The a/c, in Z5 went into a loop type maneuver, peaking at an altitude of approx. 3,000 ft, then descended and crashed 200 - 300 yards off the port bow! Both crewmen were successfully recovered. Post accident investigation revealed (plat tape analysis) that the flight deck crewman responsible for ensuring that the tow bar was properly placed in the throat of the shuttle was improperly positioned so that he could not see the tow bar. Medical records showed that the crewman was nearly blind under night flight deck conditions and that he had not had an eye examination for several years!!!

You might have some luck finding more information by reaching out to some of these enthusiasts. Good luck!

That was what I was looking for, thank you very much

@gr8dain, at the risk of an inappropriate question, were you the "the flight deck crewman responsible for ensuring that the tow bar was properly placed"?  (You said you were a final checker...)

Also mentioned in this administrative report.

No, the aircraft is hooked up by the ships company cat crew. I was assigned to squadron that owned the aircraft. My responsibility was the F-14, the plane was good, on cat launch I saw the shuttle pull away from the launch bar. I stood up to signal cancel, but I saw the aircrew was very focused on thier task at hand. I squatted down turned my back and felt the afterburners on my back. I stood back up and saw both chutes. The report I received has the rest of the details. Quite a moment, later I asked the pilot on the way to sickbay about it. "definetly worth an E coupon" was the reply. peace

Thanks, and sorry to put you on the spot!  I was just curious is all...  (being a former Prowler pilot)  And I know the yellow shirts hook things up, I just wondered if that's on the squadron final checker's list of things to scan.  Cheers.

No problem, working the flight deck was the most intense thing I've ever done. I am an adrenaline junky, nothing came close.  peace

Naval aviation mishap investigation reports are not released to the public.

No, the aircraft is hooked up by the ships company cat crew. I was assigned to squadron that owned the aircraft. My responsibility was the F-14, the plane was good, on cat launch I saw the shuttle pull away from the launch bar. I stood up to signal cancel, but I saw the aircrew was very focused on thier task at hand. I squatted down turned my back and felt the afterburners on my back. I stood back up and saw both chutes. The report I received has the rest of the details. Quite a moment, later I asked the pilot on the way to sickbay about it. "definetly worth an E coupon" was the reply. peace


text:
If I lose 90 day currency can I fly at night at all?
Specifically 14 CFR 61.57 (a) (2) states:

For the purpose of meeting the requirements of paragraph (a)(1) of this section, a person may act as a pilot in command of an aircraft under day VFR or day IFR, provided no persons or property are carried on board the aircraft, other than those necessary for the conduct of the flight.

But the exception allowing a person to act as pilot in command without currency DOES NOT exist for night VFR nor night IFR. Therefore, can a person act as pilot in command at night while regaining currency? Can you even fly solo when you have lost your currency?

For those not familiar with it offhand, what are "the requirements of paragraph (a)(1)"? Please post or link to the specific reg. Thanks, and welcome to Av.SE.

61.57(a)(2) only applies to regaining currency if you are flying a plane that requires more than one pilot crewmember. It allows a non-current pilot to act as PIC when it would otherwise be illegal in order to make it easier for a non-current pilot to regain currency. It has no relevance to a solo pilot in a plane certificated for a single pilot, since 61.57(a)(1) does not forbid a non-current pilot from acting as PIC in a plane certificated for a single pilot as long as they are not carrying passengers.
61.57(a)(1) forbids you from acting as PIC if you are carrying passengers or if the plane requires a multi-member crew:

"Except as provided in paragraph (e) of this section, no person may act as a pilot in command of an aircraft carrying passengers or of an aircraft certificated for more than one pilot flight crewmember unless that person has made at least three takeoffs and three landings within the preceding 90 days, and—"

emphasis mine.
61.57(a)(2) allows you to operate during the day with additional required crewmembers, as long as  you aren't carrying passengers or property. This makes it easier to regain currency once you have lost it if you are flying a plane that requires multiple pilot crewmembers.
61.57(b)(1) is different in that it only forbids you from acting as PIC at night if you are carrying passengers. It contains the same language as 61.57(a)(1): "[...]no person may act as pilot in command of an aircraft carrying passengers[...]" (emphasis mine), but it does not have the bit about aircraft certificated for more than one pilot crewmember. Since it doesn't have this restriction, it doesn't need a similar exception.

OK, this popped back up to the top and I reread it again... I am loath to get into another protracted discussion, but I don't see the truth in your opening statement: "61.57(a)(2) only applies to regaining currency if you are flying a plane that requires more than one pilot crewmember." Can you remind me how you arrived at this conclusion? (2) follows (1), which makes it clear that "general experience" under (a) applies to carrying passengers, OR acting as PIC in a plane requiring more than one pilot.

@MichaelHall Even if 61.57(a)(2) did not exist, it would be legal for a non-current pilot flying a single-pilot aircraft to act as PIC (solo) and regain lost day currency. But it would be illegal for a non-current pilot flying a multi-pilot aircraft to act as PIC of a multi-pilot crew. 61.57(a)(2) allows such a pilot to act as PIC. It doesn't require anything- the language used is permissive ("may", not "must"). It isn't helpful for a pilot who can just solo the aircraft since it comes with extra restrictions.

OK, we are back to where I punted before... But, I still don't agree with your conclusion that (2) applies to multi pilot only:  "it would be legal for a non-current pilot flying a single-pilot aircraft to act as PIC (solo) and regain lost day currency."  Yes, this I agree with.  And that is exactly what (2) says - while pointing out that you cannot carry passengers until you do so.  The restriction on not carrying passengers while acting as PIC if you aren't current applies to both single and multi pilot aircraft, does it not?

The way I interpret the intent of this section WRT to multi-pilot operations is that you "may" act as PIC if you aren't carrying passengers.  However, if you are carrying passengers, you can still regain landing currency as sole manipulator, but there must be another pilot acting as PIC in the aircraft.  Single pilot cannot carry passengers while acting as PIC if they aren't current, period.  Do you agree with this?

You said "But it would be illegal for a non-current pilot flying a multi-pilot aircraft to act as PIC of a multi-pilot crew."  No, it would actually be legal "provided no persons or property are carried on board the aircraft, other than those necessary for the conduct of the flight."  That is the exact verbatim caveat per section (2).

@MichaelHall It would be illegal if 61.57(a)(2) did not exist, as I said. The point of 61.57(a)(2) is to make exactly this operation legal. It has no effect on single pilot operations because the equivalent single pilot operation (solo flight with no passengers- cargo is allowed) is already legal even without it.

I don't understand the reason for "if X did not exist". It does exist. Interpretation of its intent is the central theme here.  I'm not seeing any clarification that might come from temporarily ignoring what it says.

@MichaelHall The discussion is about what purpose 61.57(a)(2) serves. Understanding what it changes (i.e. the difference between "if X did not exist" and "X exists") is key to understanding its intent. To whit, if 61.57(a)(2) did not exist, nothing would be different for a single pilot operation. This is the sense in which I mean 61.57(a)(2) only applies to multi pilot operations. It only gives some rights that single pilots already have to multi pilot operations.

I agree that (2) isn't really necessary because the requirement to get current again is implied in (1), but you cannot say that (2) does not apply to single piloted aircraft because it clearly states "For the purpose of meeting the requirements of paragraph (a)(1) of this section..."  Section (a)(1) includes all aircraft.  If the intent was that (2) only applies to multi-pilot aircraft it would instead say "For the purpose of aircraft certificated for more than one pilot flight crewmember meeting the requirements of paragraph (a)(1) of this section..."

@MichaelHall We agree that it has no relevance to a single pilot, yes? And that it has relevance to a pilot of an aircraft certificated for more than one pilot flight crewmember? Obviously the law doesn't vanish for you if you're flying solo, but it is also completely irrelevant for you.

I agree that (2) isn't necessary to understand the requirement to get current, but it is applicable to a single pilot, and as such, is relevant.  So I guess it depends on how you define "relevance"!  ;)  A single pilot still must perform the landings "under day VFR or day IFR, provided no persons or property are carried on board the aircraft, other than those necessary for the conduct of the flight."  Yes, you comply when you are solo.  Yes, it's kinda redundant and not really needed.  However, there is no exemption made for single piloted aircraft.

@MichaelHall No, a solo pilot can perform landings with cargo and at night if they like. That's my whole point. 61.57(a)(2) says that a pilot may act as PIC under those conditions, not that they must follow those conditions any time they act as PIC. It's an extra path to regaining currency, not the only one. A solo pilot outside of currency reads 61.57(a)(1), sees that they are not "carrying passengers" or flying an "aircraft certificated for more than one pilot flight crewmember," and so they can act as PIC, day or night. They read 61.57(b)(1) and see they aren't carrying passengers

so they can act as PIC at night. Once they've done 3 night landings to a full stop, they read 61.57(a)(1) again. They see they have made "three takeoffs and landings within the preceding three days," and so they are day current. They read 61.57(b)(1) again and see that they are also night current.

At no point did they need to reference 61.57(a)(2), as they weren't carrying passengers or flying an aircraft certificated for more than one pilot crewmember. And so they didn't need the extra permission that 61.57(a)(2) gives pilots to act as PIC when they aren't solo, so long as they only bring required crewmembers, fly in the day, and don't carry cargo.

"a single pilot still must" Where do you see the word must in 61.57(a)(2)?

"Must" is not in (2).  I meant must in the context of (must have) "made at least three takeoffs and three landings within the preceding 90 days" as required in (1).  Anyway, use of the word "may" in (2) muddies things in my opinion, but I stand by my assertion that (2) does not specifically or implicitly exclude single piloted aircraft.  And with that, I will punt...  (my mind is dead and my fingers are numb, but I respect your knowledge and would happily buy beers to chat more if you are ever in the Seattle area...)  Cheers.

@MichaelHall Might take you up on that if I'm ever in the area. Cheers!


text:
Can anyone identify this biplane from 1920s-1930s?
It was my grandfather's, but he is no longer around to ask. He lived in Nebraska.

The car behind looks like a Packard 12 coupe quite an upmarket vehicle especially given the economic climate of the early 30s
https://classiccars.com/listings/view/1275436/1934-packard-twelve-for-sale-in-morgantown-pennsylvania-19543


text:
What is the cargo capacity of a Boeing 737-200/-300?
I would like to know the maximum commercial load of a cargo aircraft B737-200 & 300.

@ymb1 It's a different aircraft and the title doesn't match the question. I fixed it.

@ymb1 That is probably best.

Weight, size or volume?


text:
Are Boeing 737-800s grounded?
I’m flying on a WestJet Boeing 737-800 from Honolulu to Vancouver and then on to Toronto in July, returning on the same type of aircraft in August. Are these aircraft still grounded and are they similar to the MAX 8?

This isn't a duplicate. The other question only covers the fact that the -800 and MAX 8 are different planes. That doesn't answer the question of whether the -800 is also grounded.

It really is the same question, the question of whether they are also grounded is incidental. FWIW, they are not also grounded.

The 737-800 is not grounded (and never has been). The aircraft type has an excellent safety record. Since 1997 over 5000 have been produced.
The 737 MAX 8 is its successor. The aircraft is very similar to the 737-800. One of the differences is the placement (more forward and higher) of the slightly bigger engines. This changed the stability of the aircraft under certain flight conditions, causing Boeing to introduce the Maneuvering Characteristics Augmentation System (MCAS). The MCAS played a crucial role in two recent crashes of the 737 MAX 8, leading to the grounding of the aircraft type.
The 737-800 does not have such a system and is therefore not affected.

Airlines typically use IATA aircraft codes (3 letters) or ICAO aircraft type designators (4 letters) to indicate the aircraft type used on the flights.
For the Boeing 737-800 the ICAO type designator is B738, while the IATA codes are 738 and 73H.
The Boeing 737 MAX 8 has the ICAO type designator B38M and IATA code 7M8.

@Cloud Not every Boeing is a 737 MAX; Boeing makes other aircraft types that hold excellent safety records.  And if you compare the safety records of  the 737 MAX to the safety records of the A320 two years after its entry into operation, you will see that the MAX accumulated more flight hours after its first two fatal accident (JT610 & ET302) than the A320 did (AF296 & IC605). In both A320 accidents as well as both B737 MAX accidents the pilot's awareness & understanding of the automation played big role. It's sad to see that Boeing is learning some of the same lessons again after 30 years.

Totally agree, but what I mean it Airbus has no new systems implemented recently, so it's just the safest option at the moment. If I am booked to fly on an A320 and the airline changes it last minute, I will refuse boarding and demand a refund.

The 737-800 is cleared to fly, and thousands fly every day.
Only the latest design revision, the 737-8 Max or "Max 8" are grounded, due to suspicion of a new automation system.  (and also the Max 7 and 9).
The 737 is the most-produced jetliner in the world, with 3 major design revisions and many updates over decades. That design is extremely well refined and proven.  Because of that, the newer versions like the -800 are particularly safe.
So what's all the fuss about?
The 737 itself is a refinement of earlier jets, and the "engines under wing" layout harkens back to the original production jet: the Messerschmitt 262.
Have you ever made a transition from a rear-wheel-drive car to a front-wheel-drive car?  You might have noticed when you punch the gas, the car "pulls" a little to the left. You correct that with the steering wheel, and you hardly notice because you're already making steering wheel corrections all the time. It's automatic, unconscious.
The amount of that "torque steer" varies in different conditions (slow vs fast) and different cars and different engines in the same car (4-cylinder vs V-6).  But nobody has to go back to driving school for it, do they?
Airplanes have things like that.  Particularly, engine-under-wing aircraft have the thrust line below the center of gravity... so punching the gas will make the airplane tilt upward a bit.  Just like your car's torque steer, it varies between speeds, altitudes, airplane model etc.  And just like you, the pilot corrects for it at the wheel. No big deal.

Almost all jetliners are like this.
Compared to your airplane, the 737 MAX 8 has slightly bigger engines, which makes this effect a little more.  And it's such a small change to the 737, that the (very reasonable) argument is that pilots shouldn't have to go back to flight school to learn how to fly the same airplane with a slightly bigger engine.  Unfortunately, somebody decided this little extra bit crossed a line.
They had something of a case, because under rare conditions (high nose-up and low speed and ...) those bigger engines could cause a balance issue, making the controls feel soft, and that might confuse pilots. Imagine if you're going around a cloverleaf in your car, climbing a hill at 20 mph and suddenly you need more steering wheel than you expect.  The new system (MCAS) added a bit of nose-down "trim" to help the airplane feel normal. FAA decided this was required to certify the plane.
Mind you, this part is about the MAX, not your airplane.
But when you add a whole new system, you add a whole new way for that system to fail.  We don't know the answer, and accident speculation isn't what we do here, but all the evidence so far is pointing to an unexpected problem there.
Mind you, aircraft designers expect problems and design for every failure mode they can anticipate - and they did their diligence in this system too.  It was designed to fail in a way that pilots already know how to fix*.  Apparently this hasn't worked, and "why not" is the question of the hour.
It's all part of some very hard questions being faced in aviation about whether automation makes things better or worse. We're facing these hard problems because aviation has solved all the easy problems. They just don't have accidents the way they used to.   Southwest practically proves the rule: America's safest and busiest carrier, they have had an all-737 fleet since 1971***.  They have the largest 737 fleet in the world, and each plane averages 6 takeoff/landings a day.

*  The up/down trim can be worked with either handwheels or pushing a switch that lets electric motors turn the handwheels for you.  It's always been possible for that switch to get stuck or shorted, running the trim to the extreme position.  The motors are intentionally weak and you can grab the wheels and stop/overpower them.  There is also a switch to turn them off, right on the console.  And that switch hasn't moved since 1967.
*** In the 1980s they leased a few 727's. But that's it.

Downvoter keep in mind I'm writing this for OP, who is a nervous flier. I am not writing this for aviators.  That makes me very vulnerable to highly technical nitpicking, but that is unfair and unconstructive given the audience.

I'm downvoting because this is accident speculation and has a whole paragraph about Fukushima, which is completely unrelated to the question.

This answer seems at odds with what we read in the media, which is that the crashes occurred due to a malfunction of a sensor, and that the pilots were indeed trying to compensate for it. Is the media misreporting?

@DanHulme Fukushima - gone. It barely had any accident speculation, and I've sanitized even that.  I don't know what you're looking for, honestly.

@JBentley Airplanes don't crash from "malfunction of a sensor".  A great deal more than that has to happen.  No doubt the media you're watching is "dumbing it down" for the general public.  I would suggest aviation media, Mentour Pilot on Youtube, or blancolirio on Youtube for all the gory details.

"Make it as simple as possible, but not simpler" ─ this seems to have gone past the line, I should think. This answer definitely looks like it's mischaracterizing the circumstances in which the MCAS was thought necessary, particularly given the fact that the new engine placement significantly reduced the dynamic stability of the aircraft. It's not just a quantitative change: in your analogy, it's a car where if you let the torque steering go past a small amount, then the car will spontaneously throw itself into a tailspin.

@E.P. Alright I think you're overstating the severity, but I see your point, and have edited.  It's not all the way to where you want, but it's a middle ground I hope.

@Harper I am very much aware of the difficulty of describing technical material to non-technical audiences. To my mind (with technical information mostly from here) this still isn't quite at that suitable middle ground, I'm afraid. The fact that the problem is difficult does not stop incomplete solutions from being incomplete.

@E.P. No one ever thinks that about middle ground, do they :)  I don't have credentials but I can see the URL and -- oh, that. Seen it, disregarded it, it's not even wrong.  It's popular (87 sites link to it) but it's a fact-free zone. Attention-seeking mouth-runner who's making stuff up to fit a moral parable that is sexy. (note: "what it looks like"). As bogus as the ones blaming the young pilot (data says: he had game). Ugh.  REAL truth is not sexy nor publicized. Note the view counts on that Fukushima vid, or the measly ~70k views on most of Blancolirio's stuff.

@Harper I'm uninterested in Fukushima red herrings. If you have references that explain the technical background in ways that back up your comments, I'm happy to read them and re-evaluate the claims in G. Travis's piece. As it stands, your answer looks misleading to me.

@E.P.  It's about having the ability to distinguish good information from bad.  For good technical info, try these https://www.youtube.com/playlist?list=PL6SYmp3qb3uPp1DS7fDy7I6y11MIMgnbO (they're out of chronological order).  This guy spent 2 years doing far-and-away the best reportage on the Oroville disaster and rebuild.   He started covering the MCAS issue immediately after Lion Air. His fact checking is painstaking and pristine - and he spent a lot of years flying 737NG.

The 737-800 is not grounded; it is a different aircraft to the 737 MAX 8.  The -800 was introduced in 1998; the MAX 8 in 2016.

This doesn't seem to add anything new to this answer (apart from the reference to 2016) and doesn't address the question about similarities between the aircraft (which the other answer already does).

Both crashes that led to grounding of 737 MAX were caused by malfunctions related to MCAS which is not present in the 737-800.
So if it's a safety concern you should rest assured. 737-800 has an outstanding record for safety.

This answer adds nothing that is not in DeltaLima's answer 18 hours prior.


text:
Helicopter Basics: How does lead-lag of blades actually help in compensating for increase/decrease in speed due to flapping?
I understand the following -

Due to dissymmetry of lift, flapping up and down occurs.
Since angular momentum has to be conserved, the speed of blade increases/decreases during flapping
up/down motion.
To compensate for this blade leads front by a certain
degree in the rotor plane, in advancing half and lags by a certain degree in retreating half.

My question is - what does the "compensation means"? Even after rotating the blade front by 1-2 degrees (leading), the speed of blade will remain the same after it is reached its maximum lead angle. So, what happens after that?
Is it the case that lead/lag helps in absorbing the load/vibrations by allowing some degree of freedom for blade to move by few angles?

None of these is true. The advancing/receding blade moves faster/slower relative to air so they need to have more/less angle of attack to generate balanced amount of lift. Flapping  doesn't happen in the up/down or forward/backward dimension, but in the axial dimension of the blade.

@user3528438: Yes, I understand the blade flaps up and down, thereby changing the angle of attack. What do you mean by "axial dimension of the blade"? I understand flapping happens perpendicular to the hub-plane. Where am I wrong?

No flapping is up and down vertically on the flapping hinge axis, which is horizontal and perpendicular to the blade axis.  On a two blade system the flapping hinge is the teetering hinge in the center of the hub.  The axial motion is blade incidence as set by the swash plate.  The blade flapping up and down is simply the blade following the path determined by the incidence set by the swash plate as it moves, with determines the overall rotor disc plane.  With an articulated rotor, the blade flapping range of motion is the rotor's tilt range of motion.

Blade flapping is simply the action of the tilting of the rotor disc; the blade's tip is following a particular path attached to a hub that is rigidly fixed on its vertical axis. The blade flaps up and down because its incidence/pitch is being continuously varied (or not if the stick is centered) by the swash plate.  The limits of the rotor disc's tilt range is, beyond the swash plate limits, ultimately the blade flapping travel limits.  Articulating rotors can handle this, but on a two blade teetering rotor, achieving those limits is called "Mast Bumping" and is a fatal condition.
For lead lag, the issue is you need to allow for that motion on an articulating rotor helicopter because Coriolis force makes a blade want to speed up as it flaps up relative to the other blades (actually, moving off perpendicular to the mast's axis).  When it flaps up, its center of mass moves closer to the hub than a blade on the other side that is closer to perpendicular to the mast, which makes it want to speed up, like a figure skater pulling arms in while spinning.  The lead/lag hinge provides the necessary compliance to accommodate the speeding up and slowing down tendency of the blade as its center of mass moves closer and farther from the mast as it rotates while moving up and down, so it doesn't force the rotor hub to absorb the forces.  The lead/lag damper is there to resist sudden movements of this lead/lag action that can be caused by sharp ground contact, which can lead to ground resonance.
On a teetering rotor system, you don't need lead lag hinges because you have the geometry of the teetering hinge, which is the blade flapping hinge for both blades on a single flapping axis.  The up flapping blade moves away from the hub because the hinge is above the blade axis (called an under slung hub) and this keeps the CG from moving closer to the hub significantly, eliminating most of the Coriolis forces, or reducing them to a level can be absorbed by the rotor head and blades.

Do you mean 'Conservation of Angular Momentum' rather than 'Coriolis Effect'?

Same thing.  Conservation of momentum in an object rotating about an axis.  Also called "Coriolis Force".  https://en.wikipedia.org/wiki/Coriolis_force  That said, "Coriolis Effect" tends to be associated with meteorology so I changed it to Coriolis Force, so thanks for the comment.